Tissue Distribution and Biochemical Characterization of Carboxylesterases Associated with Permethrin Resistance in a Near Isogenic Strain of Colorado Potato Beetle

The tissue distribution of carboxylesterases associated with permethrin resistance in Colorado potato beetle (Leptinotarsa decemlineata(Say)) was investigated. Most of the trans-[14C]permethrin hydrolyzing activity (i.e., 77–86%) was found in hemolymph and soluble fractions. Hemolymph fraction from the permethrin-resistant strain contained significantly higher trans-[14C]permethrin-hydrolyzing activity than the susceptible strain, indicating soluble carboxylesterases involved in permethrin resistance. Through electrophoresis and chromatofocusing procedures, the pI4.2–4.8 carboxylesterases from hemolymph were determined to be most closely associated with this aspect of permethrin resistance. These carboxylesterases from the permethrin-resistant strain showed 1.7–6.0 times higher permethrin hydrolysis activities than those from the susceptible strain. Electrophoretic comparisons and kinetic studies implied that higher activity in the permethrin-resistant strain is primary due to the overproduction of the carboxylesterases rather than any qualitative change. The overall level of permethrin hydrolysis, however, was very low (i.e., 5.6 pmol/hr/larva) even in the resistant strain. Hemolymph carboxylesterases were inhibited by permethrin in a reversible manner, suggestive of high-affinity binding of permethrin to the carboxylesterases. Therefore, a sequestration of permethrin by hemolymph carboxylesterases (e.g., pI4.2–4.8 carboxylesterases) through high-affinity binding was proposed to be a major contributing factor of permethrin resistance in the permethrin-resistant strain in addition to the low level of permethrin hydrolysis.